Miniature refrigeration system for cryothermal ablation catheter

ABSTRACT

A cryocatheter for treating cardiac arrhythmias comprises a miniature refrigeration system powered by electromagnetic radiation. The miniature refrigeration system is disposed at the tip of the catheter and electromagnetic radiation is delivered to the refrigeration system by a waveguide. The miniature refrigeration system includes an engine that converts the electromagnetic radiation into mechanical work. The engine compresses refrigerant for circulation through microchannels in the refrigeration system. Such compression is achieved by using the electromagnetic radiation to superheat a portion of a liquid mass. The superheating causes explosive boiling in the liquid to propel the mass and compress the refrigerant. While particularly advantageous in a miniature refrigeration system, the engine may also be applied to other types of devices.

RELATED APPLICATION

[0001] The present application is based upon and claims priority under35 U.S.C. §119(e) to U.S. Provisional Application No. 60/341,952, filedDec. 19, 2001, entitled LASER REFRIGERATOR, which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present application relates to cryothermal ablationcatheters, as well as to miniature refrigerators and miniature enginesfor producing mechanical force or effecting mechanical work.

[0004] 2. Description of the Related Art

[0005] Ablation catheters are commonly used to treat arrhythmias bydestroying or disrupting cardiac tissue associated with the source ofthe arrhythmias or their conductive pathways. At present, most ablationprocedures are performed using an ablation catheter in which radiofrequency (RF) current is passed through tissue contacting the cathetertip to create lesions by means of hyperthermia. Use of such RF currentinvolves risk of char and coagulum formation, particularly if thelesions created are more extensive than focal lesions, such as may berequired for circumferential lesions in the pulmonary veins. Formationof char and coagulum is ordinarily caused by poor tissue contact withthe catheter tip and creates an undesirable risk of thromboembolicstroke. Other risks of RF ablation include possible unroofing of theendothelium or producing pulmonary vein contraction.

[0006] Cryothermal ablation solves many of the problems associated withRF ablation. Destruction of tissue by freezing leaves the connectivetissue matrix intact. Lesions are created by rupturing cell membranes,and damaged cells are replaced by fibrotic tissue. There is no formationof char or coagulum, and thus the risk of thromboembolic stroke is low.Additionally, as the tissue is cooled, the catheter tip adheres to thetissue which provides improved stability.

[0007] Although cryothermal ablation provides many advantages over RFablation, it has proven difficult to implement. Cryocatheters todaytypically comprise a cooling system that provides cooling power bypumping a vaporized refrigerant through a lumen in the catheter to aJoule-Thomson expander located at the catheter distal end. The length ofthe catheter (often 1 meter or longer) and the small diameter of thelumen within the catheter (often less than 1 mm in diameter) limit theflow rate through the Joule-Thomson expander. Additionally, the pressureof the vapor returning through the catheter must be held under 1atmosphere to meet FDA requirements, which further limits the flow ratethrough the Joule-Thomson expander. The cooling power of the systemconsequently is limited to about 2 Watts, which limits the depth oftissue ablation to about 4 mm.

[0008] Accordingly, there is a need in the art for a cryocatheter thatdoes not require transport of refrigerant along the length of thecatheter so as to permit increased refrigerant flow rates. At a morefundamental level, there is a need for a miniature engine that can beadapted to, among other things, drive a miniature refrigeration systemin the tip of a cryocatheter.

SUMMARY OF THE INVENTION

[0009] The preferred embodiment of the present invention overcomesdisadvantages of conventional cryocatheters by housing a miniaturerefrigerator in the tip of a cryocatheter and powering the refrigeratorusing electromagnetic radiation delivered through a waveguide. A summaryof a preferred embodiment is provided followed by a summary of inventiveaspects.

[0010] The preferred miniature refrigerator avoids the need to transportrefrigerant through the length of the cryocatheter by housing therefrigerant circulation system entirely in the tip of the catheter. Itutilizes a unique engine that is a revolutionary breakthrough inminiaturization. The preferred engine harnesses the power of a laser byconverting electromagnetic energy into mechanical work. It provides anenormous gain in delivered power per unit volume compared to other powersources such as electric motors. Conversion of optical energy tomechanical energy is accomplished by directing laser energy through agas spring onto a free surface of a liquid mass to non-uniformity heatthe liquid mass. The heating is very rapid (e.g. <100 nsec) such thatthe portion of liquid exposed to radiation quickly reaches its superheatlimit and explosively boils. The explosion propels the remaining portionof the liquid (which functions as a piston) to adiabatically compressrefrigerant in a compression chamber. The compression results in apressure increase, thus providing a restoring force which pushes theliquid towards its original position and against the gas spring, whichallows the original position to be overshot. At the point of maximumdisplacement, the laser is fired again and the cycle repeats. Theinertia of the liquid and the compression of the vapor cause the deviceto function as an oscillator which possess a natural frequency. Theenergy lost during each oscillation is replenished by tuning therepetition rate of the laser pulses to the natural frequency. By firingthe laser at (or just after) the point of maximum displacement, resonantoperation is established, and the oscillations will persist.

[0011] Inventive aspects associated with the embodiments describedherein are abundant. In one such inventive aspect, a cryo-medicalapparatus comprises an elongated body defined between a proximal end anda distal end. A closed-cycle miniature refrigeration unit, whichincludes a compressor and at least a first heat exchanger, is disposedat the distal end. A waveguide for conducting electromagnetic energyextends from the proximal end of the elongated body to the distal end.The waveguide provides electromagnetic radiation to drive thecompressor.

[0012] In another aspect of the invention, a cryo-medical systemcomprises a cryo-apparatus which includes an elongated body definedbetween a proximal end and a distal end. A closed-cycle miniaturerefrigeration unit, which includes a compressor in at least a first heatexchanger, is disposed at the distal end. A waveguide extending from theproximal end of the catheter body to the distal end conductselectromagnetic energy to drive the compressor. A coupler coupled thesource of electromagnetic radiation to the waveguide.

[0013] A further aspect of the invention is a closed-cycle miniaturerefrigeration system comprising a compressor having a housing definingat least one chamber. A liquid piston is positioned to reciprocatewithin the chamber. A source of electromagnetic radiation energizes theliquid piston by exposing a portion of the liquid piston toelectromagnetic radiation. The source of electromagnetic radiationdrives the liquid piston to reciprocate within the chamber such that theliquid piston compresses a working fluid. A heat exchanger is incommunication with the compressor.

[0014] Yet another aspect of the invention is a medical apparatus havingan elongated body defined between a proximal end and a distal end. Anengine is disposed within the elongated body and preferably at thedistal end of the elongated body. The engine includes a housing defininga chamber and a liquid mass position within the chamber. A waveguideextends from the proximal end of the elongated body to the distal endand conducts electromagnetic radiation such that the liquid mass isheated non-uniformly.

[0015] An additional aspect of the invention is an engine comprising ahousing defining a chamber. A liquid mass is positioned to oscillatewithin the chamber at a frequency. A source of electromagnetic radiationenergizes the liquid mass by exposing a portion of a liquid mass to theradiation. The radiation causes the liquid mass to be driven at thefrequency of oscillation. Preferably, the frequency of oscillation is anatural frequency of the liquid mass in the housing.

[0016] In yet another aspect of the invention, an engine comprises ahousing defining a chamber. A liquid mass is disposed within thechamber. The source of electromagnetic radiation energizes the liquidmass by exposing a portion of the liquid mass to the electromagneticradiation. A gas spring is disposed within the chamber and within apropagation path of the electromagnetic radiation.

[0017] A further aspect of the invention is an engine which comprises ahousing defining chamber which includes first and second end sectionsand an intermediate section. A liquid mass disposed within the chamber.Each of the first and second end sections of the chamber is formed of amaterial having a low affinity for the liquid of the liquid mass, andthe intermediate section is formed of the material having a higheraffinity for the liquid of the liquid mass. A source of electromagneticradiation heats a portion of the liquid mass.

[0018] In an additional aspect of the invention, a method of oscillatinga liquid mass within a housing comprises converting a portion of theliquid mass to a gas phase to propel the remainder of the liquid masswithin the housing. A substantial portion of the gas phase portionsreconverted back to the liquid phase, and the converting andreconverting are repeated to cause the liquid mass to oscillate.

[0019] A further aspect of the invention is a method comprisingoscillating a liquid mass within a housing by converting electromagneticenergy into mechanical work and heat. The oscillations are stabilized byremoving heat such that the oscillations reach steady state.

[0020] Another aspect of the present invention involves an enginecomprising a housing having chamber wall that defines a chamber withinthe housing. A liquid piston is disposed within the chamber and has atleast one free surface not in contact with the chamber wall. A source oflaser energy is positioned to directly heat the free surface of theliquid piston. The engine also includes a gas spring and a springmechanism. The gas spring is disposed within the chamber adjacent thefree surface of the liquid piston and within the propagation path of thelaser energy. The spring mechanism is also positioned within the housingand is arranged to exert pressure on another surface of the liquidpiston. Preferably, the gas spring and spring mechanism aresymmetrically disposed relative to the liquid piston.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] These and other features, aspects and advantages of the presentinvention will be further understood with reference to preferredembodiments, which are illustrated in the accompanying drawings. Theillustrated embodiments are merely exemplary and are not intended tolimit of the scope of the present invention. The drawings of theillustrated embodiments comprise 24 figures.

[0022]FIG. 1 is schematic illustration of a cryothermal ablation systemincluding a cryocatheter, which is configured in accordance with apreferred embodiment of the present invention.

[0023]FIG. 2 is an enlarged sectional schematic view of a distal end ofthe cryocatheter.

[0024]FIG. 2A is a cross-section of the cryocatheter taken along line2A-2A of FIG. 2.

[0025]FIG. 3 is a block diagram illustrating the components of a closedloop refrigeration system disposed at the distal end of thecryocatheter.

[0026]FIG. 4A is an enlarged perspective view of a heat exchanger of therefrigeration system that is configured in accordance with a preferredmode of the refrigeration system.

[0027]FIG. 4B is an exploded perspective view of etched foils of theheat exchanger of FIG. 4A in an unformed, pre-assembled state.

[0028]FIG. 5A is an enlarged perspective view of another heat exchangerof the refrigeration system that can be used in the place of the heatexchanger illustrated in FIG. 4A. In particular, FIG. 5A illustrates astacked etched-disk heat exchanger configured in accordance with anotherpreferred mode of the refrigeration system.

[0029]FIG. 5B is a partially exploded, cross-sectional view of the heatexchanger FIG. 5A taken along line 5B-5B. For illustration purposesonly, FIG. 5B shows two disks of the stacked as spaced apart from thebody of the stack to illustrate the cross-section of an individual diskand to illustrate the structural identicalness between the disks in thestack.

[0030]FIG. 5C is a cross-sectional view of the heat exchanger of FIG. 5Ataken along line 5C-5C and illustrates an annular face of a disk in thestack.

[0031]FIG. 6A is a schematic view of the distal end of the cryocatheterand schematically illustrates a compressor engine of the refrigerationsystem.

[0032]FIG. 6B is a schematic cross-sectional view of the distal end ofthe cryocatheter taken along line 6B-6B of FIG. 6A and schematicallyillustrates the construction of the catheter proximal of the compressorengine. Only those lumens associated with the compressor engine havebeen illustrated.

[0033]FIG. 6C is a schematic cross-sectional view of the distal end ofthe cryocatheter taken along line 6C-6C of FIG. 6A and illustrates theconstruction a central part of a housing of the compressor engine.

[0034]FIG. 6D is a schematic cross-sectional view of the distal end ofthe cryocatheter taken along line 6D-6D of FIG. 6A and illustrates theconstruction of a valve mechanism of the compressor that is configuredin accordance with a preferred mode of the refrigeration system.

[0035]FIG. 7A is a sectional view that is similar to that of FIG. 6D andillustrates the construction of another valve mechanism that can be usedwith the compressor.

[0036]FIG. 7B is a cross-sectional view of the valve mechanism takenalong line 7B-7B of FIG. 7A.

[0037]FIG. 7C is an enlarged cross-sectional view of a jet valve toillustrates a variation of a valve design for the valve mechanismillustrated in FIGS. 7A and 7B.

[0038]FIG. 7D is an enlarged cross-sectional view of a vortex valve toillustrate another variation of a valve design for the valve mechanismillustrated in FIGS. 7A and 7B.

[0039]FIG. 8 is a schematic illustration of the engine that is used withthe compressor in the refrigeration system and that is constructed inaccordance with a preferred embodiment of the present invention.

[0040]FIGS. 9A through 9D are schematic sectional views of thecompressor engine of FIG. 8 shown at four different stages of anoperation cycle.

[0041]FIG. 10A is a schematic illustration of a compressor engineconfigured in accordance with another embodiment of the presentinvention.

[0042]FIG. 10B is a cross section of the compressor engine of FIG. 10Ataken along line 10B-10B.

[0043]FIG. 10C is a plan view of a distal plate of the compressionengine of FIG. 10A as viewed in the direction of section 10C-10C, andillustrates a valve mechanism that regulates fluid flow into and out ofthe compressor engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] A miniature engine is particularly well suited to function as acompressor in conjunction with a miniature refrigeration system and, inparticular, in conjunction with a miniature refrigeration systememployed with a cryoablation catheter. The illustrated preferredembodiments are thus of a cryoablation system. The miniature engine,however, can be used in a variety of other applications including, forexample, but without limitation, micro-actuators (e.g. linearactuators), micro-pumps (e.g., for drug delivery), micro-acousticalgenerator (e.g., an ultrasound transducer), micro-injectors (e.g., forinkjet and fuel injection applications), and optical switches.Additionally, the described miniature refrigeration system can be usedin other applications as well, such as, for example, but withoutlimitation, in the cooling of high power density electronics,solid-state lasers, IR sensors and similar devices. The followingdescription of the preferred embodiments thus represents only onepossible application of the engine described herein in a biomedicalapplication relating to cryoablation of cardiac tissues for thetreatment of arrhythmias.

[0045] Cryoablation System

[0046] As seen in FIG. 1, the cryothermal ablation system 100 includes asource of electromagnetic radiation 102, a cryocatheter 104, and asource of fluid coolant 106. The cryocatheter 104 has a proximal end 108and a distal end 110 and includes a miniature closed-loop refrigerationsystem 112 disposed within the catheter 104. In the illustratedembodiment, the refrigeration system is disposed at a distal end 110 ofthe catheter 104. A waveguide 114 within the catheter 104 couples thesource of electromagnetic radiation 102 to the refrigeration system 112so as to power the refrigeration system 112. The coolant source 106provides coolant to the refrigeration system 112 to remove heat build-upfrom components of the system 112 and from the refrigerant circulatingjust at the distal end of the catheter 104.

[0047] In the illustrated embodiment, a laser provides laser light topower the refrigeration system 112. Other sources of electromagneticradiation, however, can be used in other embodiments and applications.For example, electric discharge, microwave or X-ray radiation can beused to power the refrigeration system.

[0048] The source of coolant 106 preferably provides a cooled or chilledliquid coolant (e.g., saline), which flows through at least the distalend 110 of the catheter 104 when the catheter 104 is connected to thecoolant source 106. The coolant source 106 can either forms aclosed-loop cooling system with the cryocatheter 104 or an open-loopcooling system with the cryocatheter 104. In the illustrated embodiment,a closed-loop system is formed with the coolant circulating between thecatheter 104 and the coolant source 106. For this purpose, the coolantsource 106 preferably comprises a heat exchanger in order to control thetemperature of the coolant entering the catheter 104. The coolant,however, need not be chilled to temperatures required for cryoablation.The coolant rather functions to remove heat from components of theminiature refrigeration system 112, as will be described in greaterdetail.

[0049] The ablation system 100 can also include a controller 116 thatcontrols the operation of the laser 102 and the coolant supply 106 inresponse to manual control, as well as possibly in response to one ormore feedback signals from various sensors and monitors used incombination with or integrated into the cryoablation system 100. Forexample, various thermcouples and mapping electrodes can be incorporatedonto the distal end 110 of the cryocatheter 106, as known in the art, inorder to provide temperature information, to assess ablation efficacy,and to locate foci of arrhythmia prior to ablation.

[0050] Cryocatheter

[0051] As seen in FIG. 1, the cryocatheter 104 includes a handle 120having a proximal end 122 and a distal end 124, and is configured to becomfortably held by a practitioner during a treatment procedureinvolving cryoablation. A plurality of conduits, conductors, and wiresextend from the proximal end of the handle 120 for connection to thelaser 102, the coolant supply 106 and the controller 116. In theillustrated embodiment, a plurality of conduits 126 connect the coolantsupply 106 to the catheter handle 120, a wiring harness 128 connects thehandle 120 to the controller 116, and an optical coupler 130 couples thewaveguide 114 to the laser 102. These conduits, conductors, waveguideand wires extend through the handle 120 to the handle's distal end 124.

[0052] An elongated, flexible catheter body or shaft 132 extends fromthe distal end 124 of the handle 120. The catheter 104 preferably has asufficient length to be introduced into the heart (e.g., into the leftatrium or right ventricle) through a percutaneous translumenalprocedure. Moreover, for certain applications, the cryocatheter 104 canbe designed to access the left atrium in a transeptal procedure.Accordingly, the distal portion of the catheter body 132 is preferablyflexible; however, the proximal portion of the catheter body 132 can bemore rigid, as known in the art.

[0053] In the illustrated embodiment, the catheter 104 includes adeflectable distal segment 134 in order to steer the catheter and toposition the cryoablation element(s) of the catheter 104. For thispurpose, the catheter 104 includes one or more pull wires that extendsthrough the catheter body 132 from the handle 120 and that are affixedto one or more locations at the distal segment 134. The pull wire orwires are connected at their proximal ends to a manual controller 136,such as a thumb lever. The thumb lever 136 when moved tightens the pullwires to deflect the distal segment 134 of the catheter body 132, aswell known in the art. In this manner, the practitioner can steer thecatheter 104 through the vascular structure and introduce the catheterinto one of the heart chambers (e.g., into the left atrium with the aidof a transeptal sheath).

[0054] Other positioning mechanism, however, can be used with thecatheter 104, either as an alternative to or in addition to the pullwire steering mechanism. For example, the catheter can be slidablycoupled with a guidewire and, for this purpose, can include a guidewirelumen that extends at least a substantial length of the catheter. Thecatheter can be slidably coupled to the guidewire externally of thepatient's body in a “back-loading” technique after the distal end of theguidewire is first positioned at the target site. Other guidewiretracking designs may also be suitable substitutes, such as, for example,catheter devices known as “rapid exchange” or “monorail” variationswherein the guidewire is only housed within a lumen of the catheter in adistal region of the catheter. Additionally, the catheter (or guidewire)can be guided to and positioned at the target site by the use ofsub-selective sheaths for advancing the guidewire and/or catheter intothe desired location and position within the heart.

[0055] The waveguide 114 extends through the catheter body 132 from itsproximal end to the distal segment 134 of the catheter and cooperateswith the refrigeration system 112. The waveguide 114 in the illustratedembodiment comprises an optical fiber that has a core and a cladding;however, other types of waveguides (both solid and hollow) can be usedin other applications. Advantageously, the optical fiber 114 carriesoptical energy the length of the catheter 104 with minimal attenuation.

[0056] The catheter body 132 houses the waveguide 114 and includes aplurality of lumens. Each lumen extends from a proximal port at theproximal end of the catheter body to a distal port located at or nearthe distal end of the catheter. Some or all of the lumens can bearranged in a side-by-side relationship, in a coaxial relationship or inany of a wide variety of configurations that will be readily apparent toone of ordinary skill in the art.

[0057] With reference to FIGS. 2 and 2A, a first lumen 138 in theillustrated embodiment delivers coolant to the distal end 110 and asecond lumen 140 returns the coolant to the coolant source 106. Neitherof these lumens 138 140 need to be significantly insulated because thecoolant supplied and returned through these lumens has a temperaturewell above that capable of damaging tissue or blood cells. A third lumen142 houses the pull wire.

[0058] Additional lumens can be provided for additional purposes. Forexample, an additional lumen can be arranged adjacent to or coaxiallyabout the waveguide 114 and connected to a supply of cooling fluid, suchas gas, in order to cool the waveguide. In addition to or in thealternative to such cooling fluid, the first and second lumens 138, 140,which carry the liquid coolant, can be used to cool the waveguide.Further lumens can carry electrical wires or other conductors that areconnected to sensors (e.g., thermocouples, thermisters, mappingelectrodes, ultrasound imaging transducers, etc.) disposed at the distalend 110 of the catheter 104. A lumen(s) can also be provided to providean inflation medium in variations of the cryocatheter that include oneor more inflatable balloons. Aspiration, irrigation, and perfusionlumens can similarly be incorporated into the catheter body.Accordingly, in addition to the illustrated lumens, one or moreadditional lumens or conduits can be provided for additional connectionsto the distal end 110 of the catheter 104.

[0059] The catheter body 132 accordingly includes a number of internalcomponents housed within the internal structure of the body and can alsoinclude various layers over the internal structure. Any of a variety ofdifferent polymeric materials, which are known by those of skill in theart to be suitable for catheter body manufacture, can be used to formthe catheter body 132. For example, the body 132 may be formed out ofpolymers such as polyethylene, PEBAX (Atochem, France), polyimide,polyether etherketone, and the like. Additionally, the catheter body 132can also includes a biocompatible, leak-proof outer jacket formed of anyof a variety of materials, such as, for example, but without limitation,nylon, PEBAX, Teflon, or other suitable plastic or polymer materials, aswell known to those skilled in the art. The catheter preferably is madein accordance with known manufacturing techniques.

[0060] The catheter body 132 also preferably has sufficient structuralintegrity, or “stiffness,” to permit the catheter 104 to be pushedthrough the vasculature to target site without buckling or undesirablebending of the body 132. It is also desirable, however, for the body 132to be fairly flexible near its distal end 110, so that the distalsegment 134 of the catheter 104 can be navigated through tortuous bloodvessel networks. Thus, in one preferred embodiment, the body 132 of thecatheter 104 is formed from a polymer such as polyethylene or PEBAX madeto have variable stiffness along its length, with the proximal portionof the body being less flexible than the distal portion of the body.Advantageously, a body of this construction enables a user to moreeasily insert the tubular body into vascular networks. Additionally thecatheter body, or at least certain sections thereof, can be includereinforcing braid or coil incorporated into its wall structure. Thereinforcing can be formed of metal or of various polymers.

[0061] As seen in FIG. 2, the distal end 110 of the illustrated catheter104 has a heat transfer element 144 with a blunt contact tip. The distalend 110, however, can have other configurations. For example, the distalend 110 can have a shaped tip design, such as, for example a loop designthat can be expanded or manipulated as known by those of ordinary skillin the art. A shaped stylet can also be used with the catheter to varythe shape of the distal end 110 of the catheter in order to hold aparticular shape during an ablation procedure to ablate a desiredpattern (e.g., arcuate or circular), or for steering or positioningpurposes. Additionally, the distal end 110 can include a plurality ofheat transfer elements that the refrigeration system 112 cools.

[0062] Refrigeration System

[0063] With reference to FIGS. 2 and 3, the closed-cycle refrigerationsystem 112 includes at least a compressor engine 150 and a heatexchanger to cool the heat transfer element(s) 144 at the distal end 110of the catheter 104. In the illustrated embodiment, the refrigerationsystem also comprises a second heat exchanger, an expander 152, andpreferably a third heat exchanger. One of the heat exchangers functionsas a condenser 154, another heat exchanger functions as an evaporator orboiler 156, and the third heat exchanger functions as a counter-flowheat exchanger 158. A refrigerant circulates through the closed-cyclesystem.

[0064] The compressor engine 150 draws in saturated refrigerant into thecompressor 150 from an inlet side of the compressor engine 150. Thecompressor engine 150 compresses the vapor isentropically to asuperheated vapor, which then flows to the condenser 154 on an outletside of the compressor engine 150. The refrigerant vapor then enters acondenser 154, where heat is removed at constant pressure until thefluid becomes a saturated liquid. The liquid then passes through thehigh temperature side of the counter-flow heat exchanger 158 and intothe expander 152. The liquid expands adiabatically in order to bring thefluid to a lower pressure. The liquid refrigerant thence passes throughthe evaporator 156 at a constant pressure. Heat flows into theevaporator 156 from the heat transfer element(s) 144 (FIG. 2) andvaporizes the fluid to the saturated-vapor state for reentry into thecompressor 150. In particular, the liquid absorbs heat from an innersurface of the heat transfer element 144, thereby cooling the outersurface and vaporizing the liquid refrigerant within the evaporator 156.The fluid enters the low temperature side of the counter-flow heatexchanger 158 before it enters the compressor 150. The counter-flow heatexchanger 158 is used to cool further the liquid refrigerant before itenters the expander 152 and to heat the vapor before it returns to thecompressor 150.

[0065] The condenser 154 receives coolant from the catheter coolantdelivery lumen 138 and discharges it to the compressor 150 for coolingpurposes or returns it to the coolant return lumen 140. Alternatively,the condenser 154 can receive coolant from the compressor 150.

[0066] The expander 152 can include one or more valves, orifices,capillary tubes or similar types of flow restrictions. In one preferredmode, the expander 152 is a Joule-Thomson expansion device.

[0067] Each of the heat exchangers—the condenser 154, the evaporator156, the counter-flow heat exchanger 158—can be manufactured asminiature structures with high surface area using photoetchingtechnology as taught by U.S. Pat. No. 5,935,424, the disclosure of whichis hereby incorporated by reference. An example of a suitable heatexchanger structure for the counter-flow heat exchanger 158 isillustrated in FIGS. 4A and 4B while, in FIGS. 5A through 5C, an exampleof the condenser 154 is illustrated. The size, shape and relative scaleof the illustrated heat exchangers are only by way of examples, and theheat exchangers can be configured and constructed to meet specificdesign requirements, e.g., to fit within a distal end of a catheterhaving an overall outer diameter of 3 mm. Additionally, either of theillustrated heat exchanger structures can be used to form the condenser154, the evaporator 156 or the counter-flow heat exchanger 158. Theillustrated heat exchanger structures also allow for the integration oftwo or more of the refrigerator's heat exchangers 154, 156, 158 into asingle structural unit (e.g., the stacked disk structure described belowcan be configured so as to form both the condenser and the counter-flowheat exchanger).

[0068] With reference to FIGS. 4A and 4B, the counter-flow heatexchanger 158 is formed by at least two foil sheets 160, 162. Adischarge from the condenser 154 is schematically illustrated asconnecting to an inlet 164 of a microchannel 166 that is etched onto thefirst sheet 160. The second sheet 162 has a similar microchannel 168etched onto it, with an inlet 170 and an outlet 172. The inlet 170communicates with the evaporator 156 and the outlet 172 communicateswith the inlet side of the compressor engine 150. The microchannels 166,168 preferably are etched only halfway through the respective foilsheets 160, 162. The foil sheets 160, 162 can be joined (e.g., diffusionbonded) together in the orientation shown. The assembly then can berolled, as shown in FIG. 4A, to construct a cylindrical heat exchanger.

[0069]FIGS. 5A through 5C illustrate a variation of the counter-flowheat exchanger 158. In this embodiment, the heat exchanger 158 comprisesa plurality of stacked disks 174. Preferably, at least most of the disks174 have the same configuration, and end caps (not shown) close the enddisks in the stack. Each disk 174 includes a plurality of annular ribs176 that are concentrically arranged, as best seen in FIG. 5C. Aplurality of openings 178 are disposed between each pair of adjacentribs 176. When the disks 174 are stacked and joined together, as seen inFIG. 5B, the stacked assembly forms four annular flow channels 180 a,180 b, 180 c, 180 d. In each flow channel 180 a-d, the fluid flowsthrough the disk openings 178 and then into an annular space definedbetween adjacent disk ribs 176 (which ribs 176 may be of the same disk174 or of the adjacent disk 174 depending upon the flow direction). Theribs 176 and the openings 178 preferably are formed on and through thedisk 174, respectively, by photo-etching, laser-drilling, EDM(electrical discharge machining) and/or similar processes.

[0070] When this heat exchanger structure is used as the condenser 154,the inner channel 180 a preferably carries incoming coolant from thecoolant lumen 138, and the adjacent channel 180 b delivers high pressurerefrigerant from the compressor 150. The next channel 180 c carries thereturning coolant, which is delivered either to the compressor 150 forcooling purposes or to the coolant return lumen 140. The outermostchannel 180 d returns low pressure vapor to the compressor 150. Ofcourse, where this heat exchanger structure is used for other purposes,for example, as the counter-flow heat exchanger, the disk stack candefine fewer channels (e.g., two channels).

[0071] While the structure of the evaporator 156 can take the form ofeither of the heat exchanger embodiments just described, its structurepreferably corresponds to the configuration of the heat transferelement(s) 144 that contacts the targeted tissue during a cryoablationprocedure. The evaporator 156 thus is preferably configured to maximizecontact between the microchannels that form the evaporator 156 (andthrough which refrigerant passes) and the inner surface or surfaces ofthe heat transfer element(s) 144.

[0072] With reference to FIG. 6A, the compressor engine 150 of thesystem 112 includes a housing 182 that defines a chamber 184 and aliquid piston 186 that reciprocates within the chamber 184. In theillustrated embodiment, the chamber 184 has a cylindrical shape;however, other shapes are practicable. While the engine 150 can beemployed on larger scales, the inside diameter of the cylindricalchamber 184 for its application in a catheter is preferably betweenabout 50 μm and 5 mm, more preferably less than about 2 mm, and mostpreferably generally not greater than 1 mm. The small diameter cylinder184 also provides a capillary action to help maintain the integrity ofthe liquid piston 186 during operation.

[0073] The cylinder chamber 184 has sufficient length to accommodate thepiston 186 and to provide for its reciprocation in the chamber 184. Thecylinder chamber length preferably provides the piston 186 with asufficient stroke for the compressor engine 150 to compress and pump anamount of refrigerant necessary to cool the heat transfer element(s) 144to a desired temperature (e.g., to −100° C.) and to actuate the valvesof the compressor. For the present application in a cryocatheter, thelength of the engine chamber 184 is preferably less than 10 mm, and morepreferably less than about 5 mm.

[0074] The housing 182 preferably is constructed to cause the liquidpiston 186 to migrate toward a generally central position within thechamber 184 when the engine is not operating. Accordingly, differentparts of the housing walls preferably exhibit different affinities forthe liquid of the liquid piston 186. In the illustrated embodiment, thehousing comprises at least three parts that define the cylinder chamber184: a central part 188 formed by a tube having high affinity for theliquid of the liquid piston 186; a proximal part 190 formed by a tubehaving low affinity for the liquid; and a distal part 192 formed by atube also having a low affinity for the liquid. Either the material ofthe tubes or coatings on the tubes can have the desired affinities forthe liquid.

[0075] The proximal part 190 and the distal part 192 are preferably madeof thermally insulating material with an inner surface having a lowaffinity for the liquid, resulting in close to adiabatic compression andexpansion of the vapor in those chambers. One suitable material ispolytetrafluoroethylene (PTFE), available commercially as Teflon™ fromE. I. du Pont and Nemours and Company. The central part 188, in additionto having a high affinity for the liquid, preferably is made of athermally conductive material, such as, for example, copper.

[0076] The ends of the cylinder 184 also preferably have low affinitiesfor the liquid of the liquid piston 186. The proximal end of thecylinder 184 preferably is closed by the distal end of the optical fiber114 or by a lens (e.g., a collimating lens) or an intermediatetransmitter that directs the laser light into the chamber 184 throughthe proximal end. In the illustrated embodiment, the distal end of theoptical fiber 114 seals against the housing 182 at the proximal endthereof. The housing can additionally comprise a window element. In thisvariation of the housing construction, the window element would be incommunication with the optical fiber and would seal the proximal end ofthe chamber.

[0077] In the illustrated embodiment, a distal disk or plate 194 closesthe distal end of the chamber 184. The distal plate 194 includes a valvemechanism 196 that selectively permits the refrigerant to flow into andout of the chamber 184. FIGS. 6A and 6D illustrates a compressor engine150 in which one-way check valves 198 a, 198 b serve as the inlet andoutlet to the chamber 184. A suction valve 198 a permits refrigerant toflow from the evaporator 156 into the distal space of the chamber 184,and a discharge valve 198 b permits refrigerant to flow from the distalspace toward the condenser 154. Neither valve 198 a, 198 b, however,permits flow in an opposite direction, The valve plate 194 preferably isformed of a superelastic, shape memory material, such as Ni—Ti alloy,available commercially as Nitinol™. The valves 198 a, 198 b are etchedin the desired configurations. The surface of the distal plate 194 thatfaces into the chamber preferably is coated with a material that has alow affinity for the liquid of the liquid piston 186.

[0078]FIGS. 7A and 7B illustrate another form of a one-way or checkvalve that employ no moving parts. Through well known principles offluid dynamics, either a jet valve (FIG. 7C) or a vortex valve (FIG. 7D)can also provide only one-way flow from and to the compressor.

[0079] As best seen in FIG. 8, which illustrates the engine inisolation, the resulting affinity of the liquid piston 186 to centralpart 188 of the cylinder creates spaces 200, 202 on the proximal anddistal sides of the liquid piston 186, respectively. One or more gasesoccupy the proximal space. The gas can be substantially pure vapor ofthe fluid used for the piston or can be a different fluid. By selectingthe type of gas present in the proximal space, the gas spring can have alinear (or close thereto) spring constant or a non-linear springconstant. In the illustrated embodiment, such gas or gases preferablyinclude air and/or a vapor form of the liquid that forms the liquidpiston. The laser light passes from the fiber optic 114 through gas andinto the liquid rather than directly from the fiber optic 114 into theliquid. Consequently, it is preferable that the gas or vapor besubstantially transparent to the laser radiation.

[0080] The volume of the proximal space 200 is on the same order ofmagnitude as the volume of the liquid piston 186. In the illustratedembodiment, the proximal space 200 has a diameter of about 1 mm and alength of about 1 mm.

[0081] In the illustrated embodiment, the distal space 202 functions asa variable-volume compression chamber that increases and decreases involume as the piston 186 reciprocates within the chamber 184. The valvemechanism 196 regulates refrigerant flow into and out of the distalspace 202.

[0082] The gas-filled proximal space 200 functions as a gas spring. Agas spring is also formed by the combination of the distal space 202 andthe condenser 154 and the evaporator 156 that communicate with thedistal space 202. The inertia of the liquid piston 186 and thecompression of the gas springs 200, 202 constitute the typicalcomponents of an oscillator: the system 150 posses a well-definednatural frequency and is therefore capable of operating at resonance ifexcited at the right frequency. Consequently, in the presentapplication, the liquid piston can be conceptually modeled as a massdisposed between a pair of springs. This system thus will have a naturalfrequency (f_(n)) which can be approximated by equation 1:$\begin{matrix}{f_{n} = {\frac{1}{2\pi}\sqrt{\left( \frac{P_{o}}{L_{liq}\rho} \right)\left( \frac{L_{gas}}{L_{gas1}L_{gas2}} \right)}}} & \lbrack 1\rbrack\end{matrix}$

[0083] where:

[0084] P_(o) is the system average pressure;

[0085] L_(liq) is the length of the liquid piston 186;

[0086] ρ is the density of the liquid of the liquid piston 186;

[0087] L_(gas) is the combined length of the two gas springs 200, 202;

[0088] L_(gas1) is the length of proximal gas spring 200; and

[0089] L_(gas2) is the length of the distal gas spring 202.

[0090] While in the illustrated embodiment, the distal gas spring 202 isdisposed on the distal side of the piston 186, other types of springmechanism can also be used. For example, as illustrated, an elasticdiaphragm (see FIGS. 10A and 10B and associated description providedbelow) can replace the distal gas spring of the present embodiment.

[0091] As seen in FIG. 8, the housing 182 also includes a cooling jacket204 to cool at least the central part 188 of the housing 182. In theillustrated embodiment, the cooling jacket 204 includes a one or moremicrochannels cut into the central part 188, preferably using an etchingtechnique. The cooling jacket 204 receives coolant (e.g., saline) eitherfrom the catheter coolant delivery lumen 138 or from the condenser 154and returns it to the coolant return lumen 140. To further facilitateremoval of heat from the engine 150, the central part 188 of the housing182 preferably is formed of a material having a relatively high heattransfer coefficient.

[0092] Laser light energy pulses are delivered via the optical fiber 114to the free surface at the proximal end of the liquid piston 186. Forthis purpose, the waveguide can either include: (1) a focusing lens thatfocuses the light beam to a diameter substantially matching the diameterof the chamber 184 at a location near (but distal of) the proximal endof the chamber 184; or (2) a collimating lens that aligns the beamemitted from the distal end of the optical fiber 114, which has a corediameter substantially equal to the diameter of the chamber 184. In thismanner, the laser energy is directed to heat generally the entire areaof the free surface that faces the optical fiber 114.

[0093] The liquid absorbs sufficient laser energy to superheat(instantly vaporize) the liquid to a depth of at least 0.1 of thewavelength of the laser light. The absorption characteristics of theliquid material and the high energy density of the laser are such thatthe absorption results in rapid formation of a superheated layer whichconverts liquid into gas. As the liquid is vaporized, the liquid beneathis exposed to the laser light and the superheated layer effectivelymigrated further into the liquid piston 186 (like the sparks of aburning fuse migrating along the length of the fuse). The migration ofthe superheated layer is extremely fast such that the vaporized portionof the piston 186 rapidly increases the pressure within the proximalspace in a manner akin to an explosion. While vaporization is rapid, theduration of vaporization is limited by the duration of the laser pulse.Accordingly, only a small fraction of the liquid piston 186 is vaporizedby any given laser pulse. The vaporized portion preferably representsbetween about 0.05% and 5% of the liquid piston 186 by volume, and morepreferably between about 0.1% and 1% by volume. The remaining portion ofthe piston 186 (still in liquid phase) is sufficiently long to serve asa piston and to perform mechanical work (e.g., compress the fluid in thedistal space). Typically, the length of the vaporized portion of theliquid piston 186 may be greater than 50 μm, more preferably between 0.5and 5 mm, and most preferably about 1 mm.

[0094] Similarly, the liquid piston 186 should have a diametersufficient to perform its function. In general, the greater the diameterof the liquid piston 186, the more power can be produced by the engine.At some point, however, increasing the diameter of the liquid piston 186will lead to loss of capillary action, depending upon the surfacetension of the liquid and the affinity of the central part 188 therefor,leading to loss of the liquid piston's integrity during operation. Theliquid piston 186 preferably behaves generally as a “plug flow” with adefined boundary layer around its periphery. The thickness of theboundary layer will depend upon the liquid's density and viscosity andupon the system's frequency, as understood from the following equation:$\begin{matrix}{\lambda = \sqrt{\frac{2\mu}{\omega\rho}}} & \lbrack 2\rbrack\end{matrix}$

[0095] where:

[0096] λ=thickness of boundary layer

[0097] μ=viscosity of the liquid

[0098] ω=2π times the system's frequency (e.g., the natural frequency(see Equation [1]))

[0099] ρ=density of the liquid

[0100] The boundary layer in the illustrated embodiment has a thicknessλ on the order of fractions of microns. Consequently, the piston 186oscillates generally as a mass plug.

[0101] In the illustrated embodiment, where the distal space directlycommunicates with the heat exchangers of the refrigeration system 112,the liquid of the liquid piston 186 preferably is the same refrigerantused in the refrigeration system. In one preferred mode, the refrigerantliquid comprises R-134a, having the chemical formula C₂H₂F₂, a criticaltemperature of 101.2° C. and an estimated practicable superheat limit ofabout 64° C. (based upon 90% of the critical temperature in Kelvin), ascompared to its normal boiling point of −26.5° C. under standardconditions. In another preferred mode, the refrigerant liquid comprisesR-12, having the chemical formula CCl₂F₂, a critical temperature of 112°C. and an estimated practicable superheat limit of about 74° C. (basedupon 90% of the critical temperature in Kelvin), as compared to itsnormal boiling point of −30° C. under standard conditions. Additionally,the refrigerant can comprise a mixture of fluids such as R-134a, R-23,R1-4, and cryogenenic fluids such as helium, hydrogen, neon, nitrogen,and argon. The refrigerant mixture allow the refrigerator to reachtemperature as low as 70° K. as taught in U.S. Pat. No. 5,579,654,entitled CRYOSTAT REFRIGERATION SYSTEM USING MIXED REFRIGERANTS IN ACLOSED VAPOR COMPRESSOR CYCLE HAVING A FIXED FLOW RESTRICTION, whichdisclosure is hereby incorporated by reference.

[0102] A dye preferably is added to the liquid to increase absorption ofthe input optical energy. To facilitate high absorption at 1.064 μm foruse with a Nd:YAG laser, one of the following near infrared (NIR) dyescan be added to the liquid: SDA8080 and DSB6592, both availablecommercially from H. W. Sands Corp., of Jupiter, Fla. The concentrationof the dye in the refrigerant can be tailored to match the requiredoptical density (e.g., 50 μm optical density).

[0103] The laser 102, which supplies the energy to drive the compressorengine 150, produces short pulses having a duration of less than 100nanoseconds, and preferably about 50 nanoseconds to ensure rapidformation of the superheated layer and resulting gas bubble. Thefrequency of the laser pulses substantially matches the naturalfrequency of the liquid piston 186, which is, in turn, a product of thespeed of explosive vaporization and the size and mass of the liquidpiston and gas springs, as described above. Heat introduced into theoscillating system by the laser is removed by the cooling jacketdescribed above.

[0104] In the illustrated embodiment, the laser 102 is a Q-switchedsolid state Nd:YAG laser that outputs 35 Watts of optical power at awavelength of 1.064 μm. Although the preferred embodiment utilizes asolid-state Nd:YAG laser, other types of lasers can be used, includinglaser diodes and gas lasers. The Nd:YAG laser provides pulses at arepetition rate of about 20 kHz to oscillate the liquid piston 186 atits natural frequency. The energy provided per pulse preferably is about1.75 millijoules. The energy density preferably is sufficient tovaporize during a single pulse substantially the entire area of proximalliquid surface (approximately 0.8 mm²) to a depth 50 μm, starting from aliquid temperature around ambient (e.g., body temperature: 37° C.). Whenusing a Nd:YAG laser, the vaporized layer is preferably between 10 nmand 100 μm, and more preferably between 1 μm and 50 μm.

[0105] The explosion pushes the liquid piston 186 distally. The liquidpiston 186 rebounds, moves proximally, rebounds again, and then isdriven distally again by re-firing the laser 102. With correctdimensional design and operational conditions (laser pulses synchronizedwith piston oscillation), undesired losses due to vapor-liquid heat andmass transfer through the liquid-vapor surfaces can be minimized andengine efficiency maximize.

[0106] If the laser pulses are all at the same energy, the oscillationsamplitude will start small and within few oscillations (about 5 to 10)will reach steady state level. The exact number of oscillations to fullamplitude is also influenced by heat removal characteristics and otherthermophysical characteristics of the system. In the preferredembodiment, the first pulse has higher (from 2 to 5 times greater)energy then the following pulses, which helps the system reach fullscale oscillations quicker.

[0107] The operation cycle of the engine running at steady state can befurther understood by examining four sequential snapshots during theoperation cycle. With reference to FIG. 9A, the liquid piston 186 isdisposed at a generally central location within the chamber 184 and ismoving proximally at this point in the cycle for reasons that will besoon apparent. The suction valve 198 a in the distal plate 194 opens asthe piston 186 moves proximally. This movement of the piston 186 alsodraws refrigerant vapor into the distal space 202 from the evaporator156.

[0108] As seen in FIG. 9B, the laser 102 is fired when the liquid piston186 reaches it maximal displacement in the proximal direction. The laserlight, which is delivered by the optical fiber 114, passes through theproximal vapor space 200 and is absorbed in the proximal free surface ofthe liquid piston 186, which heats the liquid non-uniformly (i.e., theelectromagnetic radiation superheats a layer of the liquid withoutsignificantly heating the adjacent portion of the liquid mass). Theheating of the liquid is too fast to allow normal boiling and about 50μm on the surface is vaporized by heating to the liquid superheat limit.In the illustrated embodiment, the vaporized layer preferably representsabout 1% the liquid piston volume. Within one microsecond thesuperheated layer causes vaporization to create a large pressure rise inthe proximal space. The explosive bubble following superheating thusprovides a propulsive force to move the unvaporized remainder of theliquid piston 186.

[0109] Under the action of the high pressure in the proximal space 200,the liquid piston 186 starts moving distally, as seen in FIG. 9C. Duringthe piston's distal travel (as well as during its proximal travel), theliquid mass exhibits a plug flow profile with a defined boundary layeraround the perimeter, as noted above. Cohesive forces (e.g., viscosity),as well as its cooled temperature, tend to keep the liquid piston 186 asone continuous unit that generally moves as a monolith, thereby actingsimilar to a solid piston.

[0110] Distal movement of the piston 186 compresses the refrigerantvapor in the distal space 202 adiabatically (similar to a conventionalpositive displacement vapor compressor). The increased pressure in thedistal chamber 202 closes the suction valve 198 a and opens thedischarge valve 198 b. At least part of the kinetic energy of the movingpiston 186 is returned to the piston 186 by elastic expansion of thedistal gas spring 202, causing the liquid piston to move in the proximaldirection. The resultant restoring force helps to push the liquid piston186 toward its original position.

[0111] Additionally, once the piston 186 has reached the point of itsmaximum displacement distally, as shown in FIG. 9D, the piston 186 movesproximally. The work of expansion of the proximal chamber 200 and thecondensation of refrigerant vapor on the wall of the cooled central part188 of the housing 182 causes a pressure decrease which also has theconsequence of imparting velocity to (i.e., draws) the liquid piston 186in the proximal direction. Due to the inertia of the liquid piston 186,however, the original position of the piston is overshot and the piston186 moves toward its maximum displacement in the proximal direction. Thelaser 102 once again is fired and the cycle repeats.

[0112] In the preferred embodiment, heat is actively removed from theengine 150 to maintain the body of the liquid piston 186 below itsboiling point and to allow the explosively vaporized portion of thefluid to return to the liquid state, serving as a reusable fuel forcontinued operation. As noted above, the central part 188 of the housing182 is preferably formed from a material that is a good conductor ofheat, so as to provide a heat sink. The heat sink is constructed to havea large surface area and is preferably further cooled by a coolant(e.g., saline) that flows in or about the central part 188. The coolantreadily removes heat from the heat sink by forced convection. With watermicrochannels, forced convection can remove heat at 800 W/cm²,permitting continual operation at high power. The cooling system removesboth the heat generated by the laser beam and the heat carried by therefrigerant pumped through the refrigeration system. According to TheSecond Law of Thermodynamics, the refrigerator rejected heat is at leastits refrigeration power multiplied by the Carnot ratio of its operatingtemperatures. Stable pressure oscillations are achieved when the totalheat from the laser beam and from the refrigerator (heat pump) sectionis balanced by the heat drawn out of the engine by the coolant flow. Thecoolant flow of the illustrated embodiment is capable of removing over100 Watts of heat. The heat transfer element 144 at the catheter distalend 110 can consequently reach −100° C. and can produce cardiac tissuenecrosis to a depth of greater than 10 mm.

[0113] The combined duration of the laser pulse and the explosiveboiling is less than 2% of each cycle, and more preferably between 0.01%and 1% of each cycle. With the solid-state Nd:YAG laser in theillustrated embodiment, the period of the cycle is on the order of 50microseconds, while the combined duration of the pulse and the explosiveboiling (which occurs during the laser pulse) is in the order of 200nanoseconds. The relatively long time period of the cycle, in comparisonto the pulse duration, permits the system to react and recover beforethe next laser pulse is delivered.

[0114] Accordingly, in the present engine, non-uniform heating is byradiation onto a free surface of the liquid. Vapor spaces on each sideof the liquid mass function as gas springs to provide a restoring force,which enables the liquid mass to enter a regime of steady stateoscillations. The engine can power a compressor enormously faster andmuch smaller (e.g., 2-3 orders of magnitude faster and smaller) than aconventional compressor and has significantly more (e.g., ten timesmore) refrigeration power of a comparable Joule-Thomson expander that iscommonly used in cryocatheter today. The compressor engine thus givesthe present cryocatheter a greater cryoablation capacity (killing depth)than that of conventional cardiac ablation catheters.

[0115] While the illustrated embodiment is a cryocatheter, it isunderstood that the present refrigeration system (or engine thereof) canbe incorporated into other types of medical apparatus as well. Forexample, the present cryocooler (i.e., refrigeration system) can beincorporated into a handheld surgical ablation probe that issubstantially rigid and can be used to directly ablate cardiac tissueduring trans-thoracic or minimally invasive surgery. The probe caninclude a deflectable tip for enhanced maneuverability and preciseplacement of the heat transfer element. The engine can also be used as aminiature ultrasound source for medical imaging and treatment. The laserbubble technology enables an ultrasound actuator that is significantlysmaller than conventional piezoelectric ultrasound transducers. As aresult, imaging catheters with the present engine can be used to providedeeper views of tissue through blood vessels.

[0116] In the refrigeration cycle, the piston requires significantdisplacement in order to actuate the valves and to move refrigerantthrough the system. The piston can have a significantly smallerdisplacement when the engine is used as an ultrasound transducer and canbe oscillated at a frequency within the ultrasound range (for example,at 10 MHz to 40 MHz). Importantly, the amplitude of oscillation of thepiston roughly matches the size of the cylinder, and accordingly, theengine can be made much smaller than a conventional piezoelectrictransducer where the amplitude of oscillation represents only a 0.2%volume change of the total transducer volume. For example, the enginemay be 100 times smaller than the piezoelectric transducer used forsimilar imaging purposes.

[0117] Another embodiment of the engine compressor is illustrated inFIGS. 10A and 10B. Where appropriate, like reference numbers with asuffix “a” have been used to indicate like parts of the two embodimentsfor ease of understanding. The foregoing description thus should beunderstood to apply equally to like parts of the present embodiment.

[0118] A diaphragm 300 distal of the liquid piston 186 a separates theliquid piston 186 a from the refrigerant. This diaphragm 300 can be madeof a composite of silicon rubber and NiTi flexure. A similarmicrodiaphragm is FDA approved and is currently used in other implacabledevices.

[0119] Movement of the piston 186 a causes the diaphragm 300 to flexproximally and distally to increase and decrease, respectively, thevolume in a compressor chamber 302 that is located on the distal side ofthe diaphragm 300. Distal movement of the diaphragm 300 adiabaticallycompresses the refrigerant within the compressor chamber 302, which isdischarged from the compressor chamber 300 through the discharge valve304. Proximal movement of the diaphragm 300 draws refrigerant vapor intothe pump chamber 302 through the suction valve 306.

[0120] Water-based liquids can absorb and remove about five times moreheat than a comparable amount of refrigerant, such as R-134a. Therefore,a variation of the embodiment illustrated in FIG. 10A can utilize dyedsaline as the liquid piston of the engine.

[0121] Although this invention has been disclosed in the context ofcertain preferred embodiments and examples, it will be understood bythose skilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. In particular, while the present engine has been described inthe context of particularly preferred embodiments, the skilled artisanwill appreciate, in view of the present disclosure, that certainadvantages, features and aspects of the engine may be realized in avariety of other applications, many of which have been noted above. Forexample, while particularly useful for small-scale application (e.g.,chamber volumes of less than 2 cm³), such as the illustrated medicalapplication, the skilled artisan can readily adopt the principles andadvantages described herein to a variety of other applications,including larger scale devices. Additionally, it is contemplated thatvarious aspects and features of the invention described can be practicedseparately, combined together, or substituted for one another, and thata variety of combination and subcombinations of the features and aspectscan be made and still fall within the scope of the invention. Thus, itis intended that the scope of the present invention herein disclosedshould not be limited by the particular disclosed embodiments describedabove, but should be determined only by a fair reading of the claimsthat follow.

What is claimed is:
 1. A cryo-medical apparatus comprising: an elongatedbody defined between a proximal end and a distal end; a closed-cycleminiature refrigeration unit including a compressor and at least a firstheat exchanger disposed at the distal end; and a waveguide forconducting electromagnetic energy, the waveguide extending from theproximal end of the elongated body to the distal end and cooperatingwith the compressor so as to provide electromagnetic radiation to drivethe compressor.
 2. A cryo-medical apparatus as in claim 1, wherein theelongated body comprises a guidewire lumen.
 3. A cryo-medical apparatusas in claim 1, wherein the distal end is deflectable.
 4. A cryo-medicalapparatus as in claim 1, wherein the refrigeration unit additionallyincludes a Joule-Thomson expander disposed between the compressor andthe heat exchanger.
 5. A cryo-medical apparatus as in claim 1additionally comprising a second heat exchanger disposed adjacent atleast a portion of the chamber to remove thermal energy from thehousing.
 6. A cryo-medical apparatus as in claim 5 additionallycomprising a condenser disposed between the compressor and the firstheat exchanger.
 7. A cryo-medical apparatus as in claim 6, wherein thebody includes a plurality of lumens that extend generally from theproximal end to the distal end, and one lumen of the plurality is acoolant supply lumen that communicates with at least one of the secondheat exchanger and the condenser.
 8. A cryo-medical apparatus as inclaim 7, wherein another lumen of the plurality of lumens is a coolantreturn lumen that communicates with at least one of the second heatexchanger and the condenser.
 9. A cryo-medical system comprising: acryo-medical apparatus including an elongated body defined between aproximal end and a distal end, a closed-cycle miniature refrigerationunit including a compressor and at least a first heat exchanger disposedat the distal end, and a waveguide for conducting electromagneticenergy, the waveguide extending from the proximal end of the catheterbody to the distal end and cooperating with the compressor so as toprovide electromagnetic radiation to drive the compressor; a source ofelectromagnetic radiation; and a coupler for coupling the source ofelectromagnetic radiation to the waveguide.
 10. A cryo-medical system asin claim 9, additionally comprising a coolant supply disposed externallyof the cryo-medical apparatus, the elongated body of the cryo-medicalapparatus defining a plurality of lumens, and at least one of the lumensbeing coupled to the coolant supply.
 11. A closed-cycle miniaturerefrigeration system comprising: a compressor having a housing definingat least one chamber, a liquid piston positioned to reciprocate withinthe chamber, and a source of electromagnetic radiation energizing theliquid piston by exposing a portion of the liquid piston toelectromagnetic radiation, the source of electromagnetic radiationdriving the liquid piston to reciprocate within the chamber, the liquidpiston compressing a working fluid; and a heat exchanger communicatingwith the compressor in a manner permitting circulation of coolant fluidbetween the compressor and the heat exchanger.
 12. A miniaturerefrigeration system as in claim 11, additionally comprising aJoule-Thomson expander disposed between the compressor and the heatexchanger.
 13. A miniature refrigeration system as in claim 11, whereinthe housing comprises a valve mechanism that selectively permits ingressand egress flow into and out the chamber on one side of the liquidpiston.
 14. A miniature refrigeration system as in claim 11, whereinrefrigerant from the heat exchanger flows into the housing through thevalve mechanism such that a portion of the refrigerant in the systemfunctions as the working fluid in the compressor.
 15. A miniaturerefrigeration system as in claim 12 additionally comprising a compressorpump, the compressor pump being operatively coupled with the chambersuch that the working fluid drives the compressor pump.
 16. A miniaturerefrigeration system as in claim 15, wherein the compressor pumpcomprises a flexible diaphragm.
 17. A miniature refrigeration system asin claim 11, wherein the housing includes a cooling jacket thatsurrounds at least a portion of the chamber.
 18. A miniaturerefrigeration system as in claim 17, wherein the cooling jacket isdefined by a plurality of microchannels that communicate with a sourceof coolant.
 19. A medical apparatus comprising: an elongated bodydefined between a proximal end and a distal end; an engine disposed atthe distal end of the elongated body, the engine including a housingdefining a chamber and a liquid mass positioned within the chamber; anda waveguide for conducting electromagnetic energy, the waveguideextending from the proximal end of the elongated body to the distal endand cooperating with the engine so as to heat the liquid massnon-uniformly.
 20. A medical apparatus as in claim 19, wherein thehousing contains a gas spring disposed within the chamber and within apropagation path of the electromagnetic energy.
 21. A medical apparatusas in claim 20, wherein the engine includes a spring mechanism disposedso as to resist movement of the liquid mass away from a distal end ofthe waveguide.
 22. A medical apparatus as in claim 21, wherein thespring mechanism comprises a gas spring.
 23. A medical apparatus as inclaim 21, wherein the spring mechanism comprises a flexible diaphragm.24. A medical apparatus as in claim 19, wherein the housing includes avalve system that communicates with the chamber on one side of theliquid mass, the valve system configured to permit the ingress andegress of fluid into and out the chamber.
 25. A medical apparatus as inclaim 19 in combination with a source of electromagnetic radiation, thesource of electromagnetic radiation configured to provide pulses ofelectromagnetic radiation to a portion of the liquid mass so as to drivethe liquid mass at a frequency.
 26. An engine comprising: a housingdefining a chamber; a liquid mass positioned to oscillate within thechamber at a frequency; and a source of electromagnetic radiationenergizing the liquid mass by exposing a portion of the liquid mass toelectromagnetic radiation, the source of electromagnetic radiationdriving the liquid mass at said frequency.
 27. An engine as in claim 26,wherein the frequency of oscillation is a natural frequency ofoscillation of the liquid mass in said housing, said source deliveringpulses of electromagnetic energy at a frequency substantially equal tothe natural frequency.
 28. An engine as in claim 26, wherein the sourceof electromagnetic radiation is arranged so as to asymmetrically exposethe liquid mass to electromagnetic radiation.
 29. An engine as in claim26, wherein the source of electromagnetic radiation is arranged suchthat said portion of the liquid mass exposed to radiation includes atleast a free surface of the liquid mass.
 30. An engine as in claim 26,wherein the energy, pulse duration, energy density and wavelength ofsaid radiation are selected to cause explosive boiling of the portion ofthe liquid mass within a time period that is less than one-fourth ofsaid period of oscillation.
 31. An engine as in claim 30, wherein thetime period is on the order of 100 nano-seconds.
 32. An engine as inclaim 26, wherein the chamber has a width no greater than about 4millimeters.
 33. An engine as in claim 26, wherein the source ofelectromagnetic radiation is arranged to heat the liquid massnon-uniformly.
 34. An engine as in claim 26, wherein a vapor space isprovided within the chamber between the liquid mass and the source ofelectromagnetic radiation.
 35. An engine as in claim 34, wherein anothervapor space is provided within the chamber on the side of the liquidmass generally opposite to that on which the other vapor space occurs.36. An engine as in claim 26, wherein the source of electromagneticradiation is a laser.
 37. An engine as in claim 26, wherein the housingincludes a cooling jacket that surrounds at least a portion of thechamber.
 38. An engine as in claim 37, wherein the cooling jacket isdefined by a plurality of microchannels that communicate with a sourceof coolant.
 39. An engine comprising: a housing defining a chamber; aliquid mass disposed within the chamber; a source of electromagneticradiation energizing the liquid mass by exposing a portion of the liquidmass to electromagnetic radiation; and a gas spring disposed within thechamber and within a propagation path of the electromagnetic radiation.40. An engine comprising: a housing defining a chamber; a liquid massdisposed within the chamber; and a source of electromagnetic radiationheating a portion of the liquid mass; wherein the chamber includes firstand second end sections and an intermediate section, each of the firstand second end sections is formed of a material having a low affinityfor the liquid of the liquid mass, and the intermediate section isformed of a material having a higher affinity for the liquid of theliquid mass.
 41. A method of oscillating a liquid mass within a housingcomprising: (a) converting a portion of the liquid mass to a gas phaseto propel the remainder of the liquid mass within the housing; (b)reconverting at least a substantial portion of the gas phase portionback to a liquid phase; and (c) sequentially repeating the acts of (a)and (b) to cause the liquid mass to oscillate.
 42. A method as in claim41, wherein converting a portion of the liquid mass to a gas phasecomprises: directing electromagnetic radiation onto a surface of aliquid mass that is positioned within the housing; superheating a layerof the liquid adjacent the surface to a temperature above a boilingpoint of the liquid; and explosively vaporizing the layer of superheatedliquid.
 43. A method of converting electromagnetic radiation to kineticenergy comprising: providing a liquid mass within a chamber of ahousing; exposing a portion of the liquid mass to electromagneticradiation; vaporizing at least a portion of the liquid mass to propelthe liquid mass in a first direction; and redirecting the liquid mass ina second direction that is generally opposite the first direction.
 44. Amethod of oscillating a liquid mass within a housing comprising:converting electromagnetic energy into mechanical work and heat; andstabilizing the oscillations by removing heat such that the oscillationsreach steady state.
 45. An engine comprising: a housing having chamberwall that defines a chamber within the housing; a liquid piston disposedwithin the chamber, the liquid piston having at least one free surfacenot in contact with the chamber wall; a laser energy source positionedto directly heat the free surface of the liquid piston; a gas springpositioned within the chamber adjacent the free surface of the liquidpiston and within the propagation path of the laser energy; and a springmechanism positioned within the housing to exert pressure on anothersurface of the liquid piston.
 46. The engine as in claim 45, wherein thespring mechanism comprises a flexible diaphragm disposed adjacent saidanother surface of the liquid piston.
 47. The engine as in claim 45,wherein the spring mechanism comprises a second gas spring.
 48. Theengine as in claim 47, wherein the first and second gas springs aresymmetrically disposed relative to the liquid piston.